Back to SKKH Homepage

2.1 Earth's Crust in Motion

---

Vocabulary

---

Earthquake - the shaking that results from the movement of rock beneath earths surface
Stress- A force that makes rock change its shape or volume
Shearing - stress that pushes a mass of rock in two opposite directions
Tension - stress that stretches rock so that it becomes thinner in the middle
Compression - stress that squeezes rock until it either folds or breaks
Deformation - Any change in the volume or shape of earths crust
Fault - A breaks in earths crust where slabs of rock slip past each other
Strike-slip-fault-A type of fault where rocks on either side move past each other sideways with little- up-or-down motion
Normal fault - a type of fault where the hanging wall slides downward;caused by tension in the crust
Hanging wall - the block of rock that forms the upper half of a fault
Foot wall - The block of rock that forms the lower half of a fault
Reverse fault - A type of fault where the hanging wall slides upwards and over the footwall
fault-block mountain - A mountain that forms where a normal fault uplifts a block of rock
Folds - a bend in rock that forms where parts of earth 's crust are compressed by compression forces
Anticline - An upward fold in a rock formed by compression forces on earths crust
Syncline - A downward fold in rock formed by compression forces in earths crust
Plateau - a large area of flat land,elevated above sea level

Outline

---

Stress in the Crust

• The trembling and shaking of rock beneath the Earth's surface is called an earthquake.The movement of Earth's plates create powerful forces that squeeze or pull rock.The forces are an example of stress.Stress is a force,that deforms a rock.Because stress is a force it adds energy to the rock it squeezes or pulls.This energy is stored in the rock until it breaks or changes its shape.

Types of stress

• There are three types of stress that occur in earth's crust.Shearing,tension,and compression work over millions of years to deform rock.Shearing is a type of stress that pushes a mass of rock in two opposite directions.Tension is a stress force that streches rock until they become thinner in the middle.Compression squeezes rock until they either fold or break.The slow movement that occurs in Earth's crust is called deformation.

Kinds of faults

• A fault is created when stress that builds up in rock breaks.Faults are breaks in Earth's crust where slabs of crust slip past each other.There are three kinds of faults:strike-slip faults,normal faults,and reverse faults.

• Strike-Slip Faults
  • The stress force called shearing creates strike-slip faults where the rocks on either side of the fault slip past each other with a little up or down motion.A strike-slip fault that forms the boundary between two plates is a transform boundary.

• Normal Faults
  • A normal Fault is created by tension forces.a normal fault is at an angle so that the hanging wall lies below the footwall.When movement occurs along this fault the hanging wall slips downward. Normal faults are created where plates diverge.

• Reverse Faults
  • compression produces reverse faults.A reverse fault has a same structure as a normal fault,but the blocks move in opposite directions.When movement occurs along this fault the hanging wall slides upward and over the footwall.

Friction Along Faults

• A force that opposes one surface as it moves to another is called force.When friction near a fault is low,than the rock on both sides slip past each other without much sticking.When friction near a fault is medium than the sides of the fault jam together.When they start to jerk free they produce small earthquakes.When friction is high near a fault , the rocks lock together.Stress increases until it is strong enough to overcome this force.\

Mountain Building

• Plate movement changes Earth's surface by changing a flat plain into a towering mountain.

• Mountains Formed by Faulting
  • Normal faults are created where plates diverge by tension forces.When two normal faults come parallel to each other a block of rock is left lying in between them.When the hanging wall of the normal faults slip downward the block of rock in between moves upward creating afault block mountain.If the block of rock slides downward,a valley forms.

• Mountains Formed by Faulting
  • Folds are formed when compression forces shortens and thickens part of Earth's crust.

• Anticlines and Synclines
  • Anticlines and synclines are two tpes of folds in rock.Anticlines are folds in rock that arch upward.Synclines are folds in rock that dip downward to form a bowl.

• Plateaus
  • Plateaus are large areas of flat land elevated above sea level.

Diagram

---

This diagram shows how rock is effected by shearing,tension and compression.